WASHINGTON (CNN) -- Sens. John McCain and Barack Obama plan to appear together Saturday at a minister-moderated forum held in a church as thousands of evangelicals plan to gather in the nation's capital to pressure both men move further to the right on social issues.

Candidates are increasingly having to defend their religious views in campaigns.

TheCall -- a group representing so-called "values voters" -- will hold a rally on the National Mall as both candidates speak at Pastor Rick Warren's 20,000-member Saddleback mega-church in southern California.

The author of the best-selling book, "The Purpose Driven Life," is slated to interview both McCain and Obama.

TheCall, and Tony Perkins, president of the Family Research Council -- a Christian organization that opposes same-sex marriage and abortion rights -- will hold a press conference in Washington Friday to push McCain and Obama to delve further into issues facing evangelical voters, The Hill newspaper reported Saturday.

Former GOP presidential candidate and Baptist minister Mike Huckabee will also attend the press conference, according to the paper.

Lou Engle, Founder of the Justice House of Prayer, says "it's time for TheCall to expand to the national stage in a historic gathering in our nation's capital this Saturday," according to the group's Web site.

He says it isn't a conference or a festival, but rather a "a solemn assembly -- a gathering of all ages, races and denominations. The 12 hours of TheCall are spent primarily before the Lord in prayer and worship."

While McCain is generally considered a moderate, evangelical leaders say they do not believe he has adopted consistently conservative positions on some social issues. The Republican is against abortion rights and opposes same-sex marriage, but is a supporter of embryonic stem cell research -- a position many religious groups oppose.

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Obama's position in favor of abortion rights and same-sex civil unions have also created some tension among evangelical voters otherwise drawn to his candidacy this cycle.

But the Democrat, who is Christian, has made it a point to discuss his religion on the trail this year, and launched an ambitious outreach effort targeting these voters, including private summits with pastors and a major campaign aimed at young evangelicals.

And Obama's evangelical supporters -- including members of the newly-launched Matthew 25 political action committee, which supports his candidacy -- rallied around the Democrat when Christian conservative James Dobson accused him in late June of "deliberately distorting the traditional understanding of the Bible."

Polling suggests, however, that a majority of white evangelical voters are still backing McCain -- though enthusiasm for the presumptive Republican presidential nominee among evangelicals is less than what it was for President Bush in 2004.

In a CNN poll conducted by Opinion Research Corporation on June 4-5, nearly two-thirds of white evangelical voters surveyed, supported McCain, while 30 percent backed Obama. The poll's margin of error was plus or minus 6.5 percentage points.

By comparison, Bush received the support of 78 percent of evangelical voters in the 2004 election, according to exit polls.

But even as Huckabee -- who was thought to have locked up the evangelical vote given his background as a minister -- made a strong showing in the GOP primaries this year, McCain was pulling in a substantial number of evangelical votes.

McCain, who was raised an Episcopalian and now identifies himself as Baptist, rarely discusses his faith.

"I'm unashamed and unembarrassed about my deep faith in God. But I do not obviously try to impose my views on others," McCain said on April 11.

Since then, the Arizona senator has met with many of the evangelical leaders who did not support his candidacy during the primary season. At a private meeting earlier this summer, dozens of the movement's most prominent figures voted to support his campaign.

But by some accounts, their grassroots efforts to rally the conservative Christian base has lagged behind recently.

False rumors that Obama is a Muslim threaten to undermine support from key voting blocs like evangelicals and Catholics.

Activists organizing Saturday's event say they hope the spotlight on hot-button social issues -- which seem to have taken a back seat this cycle to worries over the economy, and concern over foreign policy crises -- will force the candidates to edge closer to their positions, or risk alienating the conservative Christian base.